On a scale from 1-10 how screwed am I?

I bought perlite from a pool supply store and it turned out it wasn’t the white balls but a white powder. I mixed it 5:1 and it seemed ok but now as it is drying it is starting to crack. Plus all the water rose to the top and I wasn’t able to smooth it out like I had hoped but I figured that sand is going on top and it wouldn’t be seen.

Not sure what to do at this point.

Ok so after sleeping on it. I thought I could fill in the cracks with the straight perlite powder as well as to mix some of it in with the sand under the firebrick. Hopefully that will provide enough insulation.

I give it about a 5. The problem is going to be with the powdered Perlite. It gets its insulative properties in pellet form from all the air spaces in each pellet. You’ve got powder form, which still has microscopic fissures in it that allow it to act as a filter medium. What is lost when it’s ground is all the spaces inside. That’s why your base shed water during curing—the Perlite powder didn’t hold onto it the way pellets would.

Spreading more Perlite powder to fill the cracks, or mixing it with the sand, won’t help with insulation. Ultimately it means your oven won’t perform as you hope, and your hard work is going to be a bit disappointing.

You can choose to go ahead, but since there are builders who report that their floors stay cold because they’re wet, I’d be doubtful about the outcome. The floor insulation is important for overall performance. It can get compromised by water, and if it’s not there in the first place, the slabs are going to act like a giant heat sink.

If it were me…I’d stew about it for a few days, then carefully break that hard mix out of the void and replace it with a mix using the specified materials. (Come to think about it, that would be dumb on my part to wait because the Portland cement is only setting up harder during those few days.)

This isn’t exactly why you put a rebar grid in your base slabs, but in this case it seems suitable.

I was under the impression that that insulating layer was to protect the concrete from the high temperatures.

I’m trying to figure out the best way to remove that layer now…ugh

A 4" grinder with a masonry blade and a respirator (safety) will remove that layer. Heck a grinder with the appropriate blade will pretty much remove any layer on this earth!

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I can understand that; but actually the opposite. The concrete slab is a large, cold mass that will absorb as much heat as it can.

I live in a home built on a concrete slab, and I can vouch for this. The floors with ceramic tile all have heat mats under them because they otherwise get unbearably cold. The floors with carpeting have thick foam pads under the carpeting which act as a thermal break. It’s not the carpeting itself that makes the surface comfortable, but the pads. A contractor laid a wooden floor across two rooms, and unfortunately neither heat nor enough insulation was installed underneath. We can’t walk on that floor barefooted in the winter.

The hearth is insulated for the same reason the firebrick arch will be later in your build: it keeps the oven’s heat from radiating away. Because convected heat rises, you have a lot of heat blasting at the arch, so you need firebrick, the insulating fiber, and pretty much anything else you can throw at it to slow down heat loss. You have conducted heat soaking into the hearth, absorbed by the firebrick. If those firebrick were directly on the concrete, they’d be like my wood floor. Heat energy would move into the colder areas (obeying the laws of thermodynamics for those of you keeping track on your physics scorecards), and the firebrick would become a conduit for passing that heat down and out of the oven.

Whether you use Vermiculite, Perlite, or insulating castable, the goal is not to protect the concrete (which cures by a thermal chemical process, and really isn’t affected by the heat—check out concrete grill pits in a state park and you’ll see that’s true). It’s to keep the oven floor hot enough to cook from below, and to allow “surplus” heat to radiate to the arch and start the convection process that cooks food evenly and quickly.

Yes, indeed. Eye protection is also mandatory, leather gloves and long sleeves because fragments will be flying, and (because I’m a musician, but it’s also a good idea in general) ear protection. For that last it’s not “just” for your hearing, but also to keep the dust out of your ear canals.

Thanks all. It is coming up easier than I had thought. Thankfully!!!

Next question is (and I can’t seem to find this conversion anywhere) how many 8qt bags of perlite would I need to equal one 4 cu ft bag?

More than you want to haul home. One 4 cubic foot bag contains about 120 quarts of product by volume. So about 15 bags.

I don’t know how much the 8 quart bag costs, but a single 4 cubic foot bag costs under $20. I’m guessing you didn’t find it readily available in your area, though it is worth calling nurseries and garden supply stores. Home Depot sometimes stocks it (that’s where I got mine), and can locate it for you. @BrickWood also names ULine as a supplier who can ship it to you quickly (big but very lightweight and they deliver quickly).

For the reasons we talked about elsewhere, you are better off doing your best to obtain the correct product and in sufficient quantity.

I’m really glad the first pour is coming out easily! Keep us posted on your progress, and good luck.

Thanks for all the input. I got it all cleaned out and I found a place locally that has perlite in a 4 cu ft bag. If anyone in the Chicagoland area is looking Brew and Grow has it.



Looking great, Jay! And thanks for sharing the Brew and Grow listing. That’s the stuff, and $25 is not out of the ballpark for it.

Referring back to your original post you should be able to screed the surface while it is still wet, and get a nice flat insulated surface ready for sand and dry-fit hearth firebrick in a couple of days. The cure time will also give you time to add your framing bricks at each end.

Thanks for keep us posted!

I think this looks a lot better.


Oh, it really does! And it will look that way when cured, except it will dry to a whitish shade.

Full steam ahead!

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